Some internal combustion engines have what are referred to as common-rail injection systems in which a plurality of injection valves are supplied with a common fuel line which is subjected to a largely uniformly high pressure. The injection quantities to be respectively injected into each cylinder of an internal combustion engine at the start of a working stroke are typically metered primarily here by the injection valves or injectors being actuated with a selected shorter or longer actuation duration during which these injection valves are opened and fuel is injected into the respective cylinder. In this context, the need to adapt actually injected injection quantities to corresponding setpoint injection quantities arises. Basically, injectors exhibit an individual quantity behavior corresponding to the fabrication tolerances. In addition, this property changes over the service life of the injector due to wear and environmental influences. In particular, wear phenomena or deposits can therefore lead to a situation in which an actual opening period or an actual degree of opening of the injection valves at a given fuel pressure and with a given actuation period changes during the service life of the injection valves. However, in order to ensure the performance and exhaust gas emissions, the variation and the service life drift which occurs must not exceed a certain amount.
When injection valves are used there are nowadays various measures for ensuring the desired accuracy. Basically, a predefined setpoint value in the form of a characteristic diagram is stored in the control unit of the injection system. Here, the injection quantity which proves to be “normal” in the new state is mapped. In addition, this characteristic diagram is adapted to the individual tolerances of the respective instance by suitable injector coding. Furthermore, different algorithms and/or correction functions which detect the quantity drift and correct it are stored in the control unit software. In this context, permissible ranges in the injector characteristic diagram are defined for the respective functions in terms of calibration. In the transition region, the correction variables are changed one into the other by interpolation. The definition of the regions is based on considerations of the possibility and effectiveness of correction of the respective adaptation function.
In order to compensate a corresponding drift of properties of an injection valve in the course of its service life, it is known, for example from published document DE 102 57 686 A1, to carry out what are referred to as minimum quantity adaptations in which the influence of injected minimum quantities of fuel on segment times of a crankshaft movement of the internal combustion engine is analyzed. Further adaptation methods for minimum quantities and for medium to relatively large quantities are also known.
However, such known adaptation methods are only ever used once. However, this has the disadvantage that methods which are configured for certain conditions, for example methods which are determined for minimum quantities, fail under other conditions, for example in the case of medium to relatively large quantities, or produce inaccurate results. The adaptation quality is therefore not particularly high.